Fix flaky dtest: transient_replication_test.TestTransientReplicationRepairStreamEntireSSTable

failed on setup with "TypeError: 'list' object is not callable"
self = <transient_replication_test.TestTransientReplicationRepairStreamEntireSSTable object at 0x7fecd5e4bda0>
fixture_dtest_setup = <dtest_setup.DTestSetup object at 0x7fecd60f8dd8>

    @pytest.fixture(scope='function', autouse=True)
    def setup_cluster(self, fixture_dtest_setup):
>       self.tokens = self.tokens()
E       TypeError: 'list' object is not callable

transient_replication_test.py:206: TypeError